The purpose of this project is to investigate the physiological roles of peptides in the nervous system, employing both invertebrate and vertebrate systems. Much of the work has been done on molluscan nervous systems, examining the electrophysiological effects of the vertebrate peptide vasopressin and analogues on membrane properties of neurosecretory cells. The peptide produces long-lasting changes in the membrane properties of specific cells characterized by the induction or enhancement of bursting pacemaker potential activity. The peptide effects are unlike those caused by conventional neurotransmitters and appear to reflect a neurohormonal role rather than a neurotransmitter role. Results obtained on the invertebrate may provide insight into one of the ways peptides may act in the mammalian central nervous system as long-term regulators of neuronal excitability over a defined population of nerve cells. A second aspect of the work has focussed on the distribution of tripeptide thyrotropin releasing hormone (TRH) in invertebrate and vertebrate nervous systems. That this peptide is present in species devoid of pituitary tissue and is located throughout the nervous system including the spinal cord strongly suggests functional roles for the peptide other than the release of thyroid stimulating hormone from the pituitary.